This invention relates to methods for reducing pollutants in the exhaust gas of multi-cylinder motor vehicle internal combustion engines.
In conventional multi-cylinder motor vehicles, an exhaust-gas purification device containing a catalyst is arranged in an exhaust line of the internal combustion engine and the air/fuel ratio of the internal combustion engine is set in such a way that a lambda value of approximately one is established in the overall exhaust flow from all the cylinders. In order to reduce the quantity of pollutants in the exhaust gas of an internal combustion engine operated with an air/fuel mixture, the exhaust gas is purified catalytically in the exhaust-gas purification device. In this process, the unburned hydrocarbons contained in the exhaust gas are oxidized to produce carbon dioxide and water, and the carbon monoxide which is also present in the exhaust gas is oxidized to produce carbon dioxide. Such oxidation processes make the presence of oxygen in the exhaust gas necessary. The nitrogen oxides produced during combustion of the air/fuel mixture in the internal combustion engine as a reaction product of oxygen and nitrogen from the air react in the catalyst of the exhaust-gas purification device with the carbon monoxide present in the exhaust gas to produce nitrogen and carbon dioxide.
Based on the fact that supplying additional air leads to a considerable reduction in the level of hydrocarbons and of carbon monoxide in the exhaust gas, it is known to supply the exhaust system with secondary air. U.S. Pat. No. 4,345,432 describes an internal combustion engine in which the emission of pollutants during the warm-up phase following a cold start is minimized by injecting secondary air into the exhaust line upstream of a catalyst device. For this purpose, a secondary air pump driven by the crankshaft of the internal combustion engine is provided, supplying air into the exhaust line in a controlled manner. However, such a secondary air pump leads to an increase in weight and reduces the useful power of the internal combustion engine.
To avoid this disadvantage, German Patent No. 40 29 672 discloses an arrangement in which the fuel supply to one cylinder of the internal combustion engine is interrupted in those operating ranges of the internal combustion engine during which the catalyst has an insufficient cleaning action, especially in the warm-up phase following a cold start, until the catalyst has reached its start-up temperature. This cylinder thus then delivers only fresh air, which passes directly to the inlet of the catalyst through the exhaust line. The fresh air causes a preliminary oxidation reaction in the untreated exhaust emissions and this preliminary reaction reduces the proportion of pollutants in the exhaust gas until the catalyst reaches its operating temperature. Deactivating an individual cylinder in this way causes temporary cooling of the cylinder wall, and this may give rise to stresses in the cylinder wall. In addition, the cooling of the cylinder wall can lead to the deposition of fuel components, especially of hydrocarbons. A further problem is that deactivating one cylinder in the case of internal combustion engines with four or fewer cylinders causes interruptions in the torque output, which are perceived as unpleasant by the occupants of the vehicle.